1. Field of the Invention
The present invention relates to a fixing device having a heat-applying device of an electromagnetic induction heat applying system, and in particular, to a fixing device and an image formation apparatus employing the fixing device.
2. Discussion of the Background Art
In an image forming apparatus, such as a copier, a printer, a facsimile, a duplicator, a multifunction machine of those, and the like, an image is created by transferring a toner image carried on a latent image carrier onto a recordation medium like a sheet. The toner image is fixed onto the recordation member due to an operation of melting and a penetration behavior of the toner subjected to heat and pressure when the toner image passes through a fixing device. As a heat applying system, a heat roller type fixing system that includes a heat-applying roller having a halogen lamp and a pressure-applying roller contacting the heat-applying roller is exemplified. Also exemplified is a film type fixing system employed to suppress calorie rather than a roller. A fixing device employing an electromagnetic heat applying system recently receives attention.
In such a system, an induction heat-applying coil is wound around a bobbin provided in a fixing roller or a heat-applying roller, and current is supplied thereto, so that over-current is generated in the heat-applying roller. As a result, the heat-applying roller is heated. In such a situation, a film can advantageously be heated directly while omitting after heat that is needed by the heat roller type fixing system, so that a prescribed temperature can immediately be obtained.
A high frequency induction heat applying apparatus including an induction heat-applying coil that receives a high frequency voltage from a high frequency power supply is known. In these days, a quick start is achieved by introducing a high frequency induction heat to a fixing device having a low calorie performance in accordance with demand of energy saving, so that a machine becomes quickly available to a user from when a power supply is turned on.
However, when a smaller size of a sheet than a prescribed heat application width is repeatedly fed though a fixing device of a low calorie type, since a sheet passage section releases the calorie to the sheet while a non-sheet passage section does not, temperature increases at the ends thereof. As a result, an image deteriorates or a lifetime of the fixing device decreases. Thus, it has been attempted to arrange a demagnetizing coil on an exciting coil so as to cancel a magnetic flux extending from the exciting coil as described in the Japanese Patent Application laid Open No. 2001-060490.
However, the technology of the Japanese Patent Application Laid Open No. 2001-060490 can only handle a limited number of definite shape sizes. Specifically, when a demagnetizing coil shape or size is determined to handle a post card size and a sheet larger than the same like B5 (Japanese Industrial Standard) is fed longitudinally while controlling a demagnetizing amount so that the maximum temperature can be less than a prescribed level at the non-sheet passage section, temperature decreases at the ends of the sheet.
As a result, a fixing performance is defective. Otherwise, brilliance becomes uneven for the same reason resulting in creating an uncomfortable image. That is, since a heat conducting cross section reduces in the fixing device and a heat flattening performance deteriorates in a direction in parallel to an axis of a rotation member, the above-mentioned problem becomes prominent. Further, since the temperature increases at both ends, an elastic member and a protecting film and the like arranged on the heat-applying roller can be damaged.